For supplying electronic devices with power, it is known to utilize power supplies which convert an alternating current (AC) voltage, e.g., a mains voltage of 230 V or 120 V, into a lower direct current (DC) voltage, e.g., a DC voltage of 3.3 V, 5 V, 9 V, or 12 V, as required for supplying various electronic devices. For this purpose, the power supply may include a converter circuit which is based on a bridge rectifier, signal filters and a DC-DC converter. The circuit components typically include one or more capacitors which are charged during operation of the power supply circuit.
The charging of the capacitor(s) may have the effect that, when the user plugs off the power supply, the capacitor may be charged at a high voltage level of 100 V or more, which may constitute a significant risk for a person touching a contact of the power supply, because the capacitor may then discharge through the person's body. In view of this situation, security standards have been implemented which require that the power supply is equipped with a mechanism to quickly discharge the capacitor after the power supply being plugged off.
Such mechanism may involve passive discharging through a resistor which is permanently connected in parallel to the capacitor. However, this mechanism may result in increased power consumption, especially when the power supply operates under low load or no load, because there is additional current dissipation in the resistor. Further, such mechanism may involve active discharging by activating a discharge path in response to detecting that the AC voltage is no longer present at the input of the power supply. Also in this case, power consumption increases due to the monitoring of the AC voltage at the input of the power supply circuit.
Accordingly, there is a need for techniques which allow efficient discharging of an internal capacitive element of a power supply circuit.